This invention relates to methods of making electrical contacts and electrical contact materials which are typically used in medium voltage switching apparatus. More particularly, this invention concerns a method of making electrical contacts and contact materials containing zirconium diboride.
To be suitable for the above applications, a contact material should have high thermal conductivity, high electrical conductivity, high resistance to corrosion, high mechanical strength, low contact resistance, good electrical arc interruption capabilities, and minimal tendency for interfacial welding or sticking when employed in an electrical switching device.
It is known to make electrical contacts from a conductive material and a second material that tends to inhibit welding by weld embrittlement and to strengthen the composite. The conductive material, typically silver or copper, imparts high electrical and thermal conductivity to the contact while the refractory material contributes to the desirable properties of weld resistance, arc extinguishing, resistance to arc erosion, and increased mechanical strength of the composite.
Silver-tungsten composites are widely used as electrical contact materials in medium load circuit breakers. While these materials perform adequately with respect to arc erosion and weld inhibition, their oxidation resistance is relatively poor. During use, the interfacial resistance between opposed contact faces can progressively increase due to the formation of a semi-insulating surface layer of tungsten oxides and silver tungstates.
Composites employing silver or copper as the matrix component together with various carbides or borides as the refractory component have been evaluated as replacement electrical contact materials for silver-tungsten contacts because of their higher hardness, heats of vaporization, and resistance to oxidation. However, at elevated temperatures, most metal borides have a high chemical affinity for oxygen and as a result they have not found wide use as the refractory component of contact materials. In the cases of the borides of titanium, zirconium, and hafnium, for example, there is an initial, rapid reaction with oxygen to form a layer of oxide film on the surface of the boride particles. These films possess a high surface energy toward liquid silver. The presence of a passivating oxide layer on the surface of the boride particles precludes the preparation of composites with a strong silver-boride particle bond. The oxide layer easily forms and, once formed, is difficult to remove. Electrical contacts fabricated of silver and metallic borides in which a surface layer of oxide has formed on the boride during fabrication are generally porous, mechanically weak, and subject to more frequent failure under conditions encountered in an electrical switching application.
Thus, while silver-metal boride composites afford an attractive alternative to silver-tungsten composites as electrical contact materials, they are difficult to form into contacts having the requisite density and strength.
Various techniques have been employed in the fabrication of silver or copper based boride-containing electrical contacts and contact materials to inhibit the formation of an oxide layer on the borides, including vacuum sintering. These techniques, however, have met with varying degrees of success.
It is therefore an object of the present invention to provide an improved method of forming electrical contacts containing zirconium diboride.
It is another object of the present invention to provide powder mixtures suitable for compacting and sintering to produce electrical contacts containing zirconium diboride.